ap-4-12.dvi Acta Polytechnica Vol. 52 No. 4/2012 Lifetime Assessment of a Steam Pipeline Jǐŕı Janovec1, Daniela Poláchová1, Michal Junek1 1 CTU in Prague, Faculty of Mechanical Engineering, Department of Materials Engineering, Karlovo náměst́ı 13, 121 35 Prague 2, Czech Republic Correspondence to: Jiri.Janovec@fs.cvut.cz Abstract The aim of this paper is to design a method for assessing the life of steam pipes for Czech power plants. The most widely-used material in Czech power plants is steel 15 128. Our findings may also be applied for international equivalents of this steel. The paper shows the classification of cavitation damage and microstructure classification status, based on the German VGB Act, with references to EPRI law in the USA. Calculations of remaining life on the basis of Russian experience are also shown. The possibility of applying this method to increase the operating parameters for power plants is discussed. Keywords: life assessment of a steam pipeline, creep damage, assessment of microstructure damage. 1 Introduction A switchover to high-parametric power plants re- quires an assessment of the remaining life of exist- ing steam piping. In creep conditions, this requires a large amount of information on operating condi- tions (temperature, pressure, time), material charac- teristics (microstructure, creep strength, creep strain rate, etc.) It is also important to monitor the stress in exposed parts and weld joints. The greatest influ- ence on component life is the formation and joining of internal defects, i.e. cavities. It is therefore necessary to provide a sophisticated method for observing and evaluating internal defects. This paper deals mainly with steel type 15 128, because the vast majority of the steam piping currently operating in the Czech Republic is made from this material. 2 Material 15 128 (14MOV6-3) This is a low-alloy heat-resistant CrMoV steel with guaranteed weldability. The main use of this steel is for steam piping, superheaters and boiler tubes operating at temperatures up to 580 ◦C. The microstructure of steel in the initial state de- pends on the heat treatment that it has undergone. The initial state of 15 128.5 has a ferritic-bainitic mi- crostructure with fine dispersion of globular carbides of M4C3 or MC type, respectively, precipitated in the ferritic matrix. The initial state of 15 128.9 is formed by a fine carbide-bainitic microstructure. The two initial states differ in their mechanical properties; see Table 3 [1]. Table 1: Equivalent materials according to international standards ČSN DIN GOST ASTM W. nr. marking 15 128 1.771 5 14MoV6-3 12Ch1MF Gr. P24 Table 2: Chemical composition of the material according to the standard Identification according to ČSN Chemical composition [weight %] C Mn Si Cr Mo V P S Al 15 128 0.10 0.45 0.15 0.50 0.40 0.22 max. max. max. 0.18 0.70 0.40 0.75 0.60 0.35 0.040 0.040 0.025 74 Acta Polytechnica Vol. 52 No. 4/2012 Table 3: Mechanical properties of steel 15 128 in dependence on heat treatment State Rm Rp0,2 A5 HB RmT /10 5 hrs [MPa] RmT /2.5 · 105 hrs [MPa] *) [MPa] [MPa] [%] 550 ◦C 575 ◦C 600 ◦C 550 ◦C 575 ◦C 600 ◦C .5 490 690 min. 365 min. 18 140 197 89 64 45 73 51 35 .9 570 740 min. 430 min. 17 163 223 107 75 (51) 88 59 (38) *) 15 128.5–960 ◦C/30 min/air + 710 ◦C/1hour/air 15 128.9–970 ◦C/30 min/water + 720 ◦C/1hour/air Table 4: Summary of tests for steam piping [2] Type of samples Test methodology Obtained data Pipeline (non-destructive tests) Replicas Visual tests Cavity formation Cracking Ultrasonic methods Hardness testing Density of inclusions Spheroidization Penetration test Mag. powder test Segregation of sulphides Hardness Optical fibre measurement Chemical analysis Size and type of grain Metallographical samples (destructive tests) Replicas Hardness testing Fusion line cracking Cavities formation Light microscopy Electron microscopy Hardness Spheroidization Cryo-cracking Cross-weld stress rupture test Fine-grain HAZ damage Damage to weld metal Density of inclusions Size and type of grain 3 Monitoring the operating parameters, service life management The main parameters that should be monitored over time are the temperature and the pressure of steam in the steam pipeline. Regular monitoring of the op- erating parameters several times a year is most ad- vantageous in terms of creep lifetime. 3.1 According to epri (USA) [2] A three-stage approach is used for evaluating the life- time of steam pipelines. At each stage, the estimated remaining life and the desired service life of the steam pipeline are compared. STAGE 1: Includes general calculations based on operating history and especially exploring the pos- sibility of degradation of components. The main con- tents of this stage are the relevant technical drawings, material properties, operating hours and cycles, his- tory of inspections and maintenance, failure history (details of failures and repairs to failures), opera- tional parameters and their maximum values (tem- perature and pressure). STAGE 2: Includes non-destructive testing of components, the results of which can improve the evaluation of the lifetime in STAGE 1. For tests carried out at this stage, see Table 4. Vi- sual examination includes observation of geomet- ric inaccuracies (e.g. buckling). This includes ge- ometry measurements (wall thickness, ovality) and measurements of the position of selected hinges and supports. If cracks are found by capillary tests, an ultrasonic examination is to be made. The replica method provides a preliminary esti- mate of the lifetime of the steam piping (welded joints). 75 Acta Polytechnica Vol. 52 No. 4/2012 Figure 1: Creep curve with states of cavitation damage marked on it [3] STAGE 3: Includes destructive testing and de- tailed analysis of the samples. It is necessary to in- terrupt the operation and remove a part of the steam pipeline. This stage provides the most accurate es- timate of the remaining life of the samples. The ob- served data is compared with the data determined in STAGE 2. The tests are presented in Table 4. The high cost of this type of evaluation needs to be taken into account. (For example, is it better to replace or repair a part of the steam piping, or is it better to operate under lower conditions?) According to [3], the replica method can be used on properly prepared surfaces. Two to three replicas should be taken from each site. Microstructure and cavitation (creep) damage is evaluated on the basis of an evaluation of the replicas. Figure 1 shows a creep curve with states of cavitation damage marked on it. The structural condition is assessed according to the etalons. Resistance to corrosion attack can also be assessed with the use of electrochemical polarization measurements. 3.2 Russian approach (material 12CH1MF) 3.2.1 Operation and inspection On the straight parts of steam pipelines operating at temperatures from 450 to 545 ◦C it is necessary to measure the residual deformation 200 thousand hours after the pipeline came into operation. For steam pipelines operating at temperatures from 546 to 570 ◦C, analogous measurements should be per- formed after 150 thousand hours. If the residual de- formation exceeds 0.75 %, an assessment of the ma- terial is made in terms of mechanical properties and chemical composition. On the bent parts of the steam piping, measure- ments of the residual deformation, magnetic powder and ultrasonic flaw detection must be performed af- ter 150 thousand hours. In operating temperature ranges from 546 to 570 ◦C, this measurement is per- formed after 100 thousand hours. For welded joints, the degree of fatigue life τho/τlp is evaluated in accordance with the range of struc- tures and their microdamange for welded joints (τho – hours of operation; τls – limit service life at the stage of microcrack discovery). The residual ser- vice life (τrs) can be calculated from the difference τrs = τls − τho [4]. Typical damaged places of welded joints in the creep region are found mainly along the outside of the heat-affected zone of the material. 3.2.2 Service life calculation The lifetime of a steam pipeline operated in the creep region is assessed according to the degree of micro- damage within the structure of the material. The ba- sic parameter for calculating the lifetime is the num- ber of micropores in a unit area of the metallographic scratch pattern (replica). Figure 2 shows an exam- ple of data processing for steel 12Ch1MF at 600 ◦C. For a reliable extrapolation of long-term strength at 100 000 hrs, it is necessary to start from an experi- ment planned for at least 4 000–5000 hrs. In [5], the calculation was based on 50-300 thousand hrs in creep conditions, steam piping energy blocks 250–800 MW at 515–560◦C and 3.7 to 25.5 MPa. 76 Acta Polytechnica Vol. 52 No. 4/2012 Figure 2: Example of experimental data processing in double logarithmic coordinates after tests on steel 12Ch1MF for term strength at 600 ◦C [6] 3.2.3 Creep time The creep time from pore creation to the develop- ment of a macro-crack for 12Ch1MF can be one half of the total operating time, which is (1–3) × 105 hrs (in the course of a year the plant works for (7–8) × 103 hrs). Information on the degree of harmfulness of the metal therefore allows the uptime capacity to be calculated. At the present time, the diagnostics is carried out mostly by metallographic methods when the energy blocks are shut down, or during an over- haul. The high work difficulty involved in preparing the metallographic sample reduces the control perfor- mance, and extensive repairs are needed. It is there- fore necessary to introduce more progressive express methods. 4 Increasing the operating parameters The Russian literature [4,7], presents examples of ser- vice life extension by up to an additional 100 thou- sand hours after appropriate repairs to the welds on the basis of microstructure analysis and degree of damage. Cyclic heat treatment (3 to 10 cycles) with heating up to 980 ◦C and cooling effectively re- generates the operating characteristics if the mate- rial damage does not exceed 10–15 % in relation to the original state, when Δρ/ρ ≤ 0.1-0.2 % [?]. For 12Ch1MF steel with 0.13 % damage just 2–3 cycles; at 0.5 % damage, 4–6 cycles; 1 % (total damage state), 8–10 cycles. Other specific cases for increasing the operational parameters are very difficult to access. Most of the results and experience gained in relation to this issue are covered by trade secrecy. However, it is obvi- ous that even a small change in operating parame- ters (e.g. a temperature increase) has a significantly negative effect on the residual service life. 5 Assessment of microstructure damage In our proposal for the metallographic evaluation of a structure, we utilize the classification degrees used by Neubauer, ISQ, and VGB Nordtest, a detailed description of which, and a comparison, are given in Table 5 [3]. Figure 3 shows various states of material 14MoV6-3 damage. Table 5: Microstructure rating class [3] Nordtest NT NDT010 VGB TW507 Neuber and Waddel Description Recommendation (Nordtest) Consumed Life-Fraction (EPRI) 0 As-received 1 1 No creep cavitation None 0–0.14 2 A Single cavities Re-examine after 20 000 hrs 0.05–0.47 2a Isolated cavities 2b Numerous cavities, no preferred orientation 3 Coherent cavities Re-examine after 15 000 hrs 3a B Numerous oriented cavities 0.27–0.53 3b Chain of cavities 4 4 C Creep cracks (micro) Re-examine after 10 000 hrs 0.29–0.84 5 5 D Creep macrocracks Issue immediate warning 0.7–1.0 77 Acta Polytechnica Vol. 52 No. 4/2012 Assessment class 1: No creep cavitation Assessment class 2a: Single cavities Assessment class 2b: Numerous cavities Assessment class 3a: Numerous oriented cavities Assessment class 3b: Chain of cavities Assessment class 4: Creep microcracks Figure 3: States of material 14MoV6-3 damage [8] 6 Conclusion Our proposal for Czech power plants anticipates a shift in service life from grade 3a to 3b, i.e. to al- lowed length of string cavitation 100 μm at a density of 20 microcracks/mm2 for steel 15 128 or its inter- national EN equivalents. The proposal is based on an experimental evaluation of microstructures, creep behavior and changes in wall thickness of bends and pipes at 6 CEZ power plants operated for a period of 25 to 35 years. Another topic for research is the design of checks on the service life of steam pipelines in the area of creep damage and damage assessment of the mi- crostructures of grade 15 steels and the new class of T23, T24, P91, P92 and E911 steels. 78 Acta Polytechnica Vol. 52 No. 4/2012 References [1] Svobodová, M., Tůmová, D., Čmakal, J.: Dı́lč́ı odborná rešerše “Přehled materiálových vlastnost́ı oceli 15 128 z databáze UJP PRAHA, a. s.”. [Zpráva UJP 1450] UJP PRAHA, a. s., Praha : listopad 2011, 22 s. [2] Fossil Plant High-Energy Piping Damage: The- ory and Practice, Volume 1: Piping Fundamen- tals. EPRI, Palo Alto, CA: 2007. 1012201.K. [3] Mentl, V.: Řı́zeńı životnosti parovod̊u v ener- getice. [Technická zpráva – Rešerše č. 16/2011/ KMM] KMM FS ZČU v Plzni, Plzeň : listopad 2011, 239 s. 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